1. Field of the Invention
The present invention relates to optical transmission and optical measurements, particularly to an optical filter-coupler which uses optical fibers or other types of waveguides for propagating optical radiation.
2. Prior Art
In conventional optical filter-couplers, the same material is used for closely placed optical fibers or waveguides.
The splitting ratio of an optical filter-coupler is defined by a function of the coupling coefficient, the difference in the propagation constant between cores or waveguides of the optical filter-coupler, and the coupling length. If the coupling coefficient is C, a half of the difference in the propagation constant is .DELTA., and a coupling length from the start of coupling is Z, the splitting ratio R, which represents how much of the light entering an incident port 1 of an optical coupler, such as shown in FIG. 1, couples from a core 3 thereof to another closely placed core 4, is given by a logarithm of the ratio of light intensities in the cores 3 and 4, Pb/Pa, as follows: ##EQU1##
Conventionally, closely placed waveguides are made of the same material, therefore the difference in propagation constant between the waveguides should theoretically be zero (0). But practically such difference in propagation constant can not be reduced to zero. This is because the closely placed cores or waveguides can not be manufactured in precisely the same diameter, consequently a slight difference in the propagation constant between the waveguides occurs. Therefore the maximum splitting ratio in such filter-couplers is given by: ##EQU2##
As shown in FIG. 2, the splitting ratio is thus governed by a periodic function of the coupling length, and therefore a major part of the power of the propagating optical radiation such as light transfers from one core to the other at a coupling length ##EQU3## (n is an integer).
Recently, there are demands in the field of multiple wavelengths transmission for wavelength filters which perfectly separate a designated wavelength. However, for example, when light of a wavelength A or light of a wavelength B as shown in FIG. 3 are desired to be completely separated, as can be calculated from the equation (2), the splitting ratio R which can be obtained by a conventional filter-coupler only reaches finite levels as shown in FIG. 3 due to the difference in propagation constant between waveguides. In other words, a complete power transfer can not be obtained by use of a conventional filter-coupler which uses the same material for its waveguide portions. Further, it is extremely difficult to manufacture an optical waveguide coupler having its maximum splitting ratios at two different wavelengths because slight changes in coupling length cause large changes in the splitting ratio.